Field of the Invention
This invention relates to oxygen sensors, and, more particularly, to an oxygen sensor suitable for use in monitoring the concentration of oxygen in gases containing minute solid particles, such as exhaust emissions of motor vehicles.
One type of an oxygen sensor known in the art is based on the principles of oxygen concentration cell or oxygen pump in solid electrolytes, such as zirconia. Another type known in the art uses the phenomenon of transmission of oxygen ions in metal oxides, such as titania. It is well known that these oxygen sensors of the prior art are temperature-dependent. Proposals have been made to provide improvements to these temperature-dependent oxygen sensors, with one such proposal disclosed, for example, in Japanese Laid Open Application No. 166252/83 including the provision of a heating element located close to the sensor element to minimize influences which might be exerted on the sensor by the temperature of gases to be tested.
In order to achieve improved fuel efficiency and engine performance while avoiding environmental pollution by exhaust emissions by effecting finer control of fuel-air mixtures supplied to the engine of an automotive vehicle, there has been a demand to widen the range of operation of the oxygen sensors of the type referred to hereinbefore from the so-called theoretical air-fuel ratio to the entire range of working air-fuel ratios, particularly to lean mixtures. To meet this demand, it is necessary that the sensor element be controlled at a higher temperature (at a constant value in the range between 300.degree. C. and about 800.degree. C. at which the point of theoretical air-fuel ratio is sensed), in view of the physical properties of its material. The exhaust pipe of an automotive vehicle in which the oxygen sensor is mounted shows great fluctuations in temperature between -50.degree. and +800.degree. C. and in the velocity of air currents between 0 and 100 m/sec. This makes it necessary to pass an electric current to the oxygen sensor to heat same nearly at all times. However, to achieve economy in fuel consumption, it would be necessary to minimize electric power used for the purpose of heating. This would make it necessary to minimize the thermal capacity of the sensor and adopt support means which could inhibit transfer of heat to a portion of the exhaust pipe in which the sensor is mounted. As a result, the sensor including a support member has generally become elongated in shape, and proposals have been made to use a sensor element of small volume which may be either plate-like, disc-like or film-like in form, as disclosed, for example, in Japanese Patent Application Laid-Open No. 42965/83.
On the other hand, as noted in Automotive Vehicle Technology, 1972, Vol. 26, No. 9, exhaust emissions contain solid particles of carbon, magnesium, silicon, phosphorus, sulfur, calcium, chromium, iron, zinc, lead, etc., existing in the form of grit produced by the combustion reaction between sucked air and fuel or lubricant and the sliding contact between engine cylinders and pistons and between suction or discharge valves and the valve seats, and these solid particles are known to be deposited on walls and other parts of the exhaust pipes. In these deposits, lead, zinc, iron, chromium and calcium, relatively heavy in weight, remain in a solid state without vaporizing even if they are heated to 800.degree. C. Thus, these elements remain deposited on the surface of the sensor element even if the latter is heated to a temperature of 800.degree. C., making it impossible for the sensor element to perform monitoring due to the fact that the bores on the surface of the element for diffusing gases are blocked by the deposits or the triphasic interface of an electrode is covered with them. It is known that the smaller the size of the sensor element, the greater the influences exerted by the solid particles on the results achieved by the sensor.
This invention has as its object the provision of an oxygen sensor capable of maintaining its capacity to monitor the concentration of oxygen at a desired level over a very long time even if it is installed in a gas flow which contains solid particles and shows great fluctuations in temperature and flow velocity.
To accomplish the aforesaid object, the invention provides an oxygen sensor capable of reducing or minimizing influences which might be directly exerted by the gas flow on the sensor element for monitoring the concentration of oxygen in the gas.
According to the invention, the sensor element is enclosed by double cylinders including an outer cylindrical member and an inner cylindrical member, wherein a gas flow, admitted to the interior of the outer cylindrical member through openings formed in the outer cylindrical member, impinges on an outer wall surface of the inner cylindrical member so that solid particles in the gas are deposited thereon, and the gas flow is admitted to the interior of the inner cylindrical member through openings formed in the inner cylindrical member in positions in which the gas flow is out of alignment with the sensor element, to thereby reduce the flow velocity of the gas flow in a vicinity of the sensor element and render the latter imprevious to the influences of the gas flow.
According to further features of the invention, the sensor element is housed in a protective metal member of substantially cylindrical configuration which is closed at the bottom and formed with openings to allow a gas flow to enter into and exit from the protective metal member in positions remote from the sensor element in the protective metal member, whereby the majority of the gas flow entering the protective metal member passes through the vicinity of the supporting portion of the sensor which is near the inner wall surface of a passage of the tested gases and direct influences which might be exerted on the sensor element by the gas flow can be reduced.